1. Technical Field
The present disclosure relates to the technical field of the design of cores for foundry parts to be subsequently forged.
2. Description of the Related Art
The core, in foundry, enables to form hollow shapes in foundry parts. It is generally made of sand or of salt.
For a better understanding of aspects of the invention, the different technologies used to design different types of cores will be briefly reminded, with their limits, in relation with the drawings.
FIGS. 1A-1B show a blown sand core or a blown salt core before and after assembly of the particles.
According to this first implementation, the sand (1) is coated with a bonding agent (2) which hardens on shooting of the core. The sand and the bonding agent are introduced into a nozzle and air is pressurized upstream of this nozzle. The core box is located downstream. The pressure is released and projects the sand into the core box, which may be hot or cold. The sand fills the core box cavity and is set by the bonding agent. The filling of complex shapes is difficult to adjust. Also, in certain cases of use of two sand supply points in the core box, shape and section variations should be limited. However, in this implementation, the sand grains are not deformed and a grain cluster is obtained (FIG. 1B) with porosities (p).
In the case of a blown salt core implementing the same process, the operation takes place in similar conditions, with the same constraints and disadvantages, especially regarding porosity.
Another known solution is that of the sintered salt core illustrated in FIGS. 2A-2B.
In this case, the salt core manufacturing technique comprises a first coining operation followed by a sintering of a mixture of a salt powder with a bonding agent and a mould-release agent. The salt grains are designated with reference (3) and the grain bonds obtained according to the process are designated with reference (4). The coining provides a core which is sufficiently solid to be manipulated. Its solidity is completed after a high-temperature sintering operation. During the sintering, the bonding agent is in a semi-solid or liquid state and fills part of the porosities persisting after the coining.
After cooling, the bonds created by the bonding agent give the core a high breakage resistance but the compressibility remains high since not all porosities (p) are filled. Further, the coining operation results in shape and size limitations to be able to form the core.
The use of the solutions implemented according to prior art thus has limits, with a degree of porosity of the obtained core capable of having adverse effects on casting of the foundry material.
Further, the core obtained according to the previously-mentioned processes, and by their heterogeneous structure, is partially inappropriate for other processing, such as for example a forging operation.
Independently from the foundry core issue, the Applicant is the designer of the COBAPRESS process (registered trademark) defined in European patent 119 365.
This process implements, for aluminum alloys, two successive casting operations to obtain a preform, which is then placed in a forging die to be forged. This technology is very widely exploited and developed by the Applicant, but also by others, since patent EP 119 365 belongs to the public domain.
The Applicant has also developed many improvements to the basic technology of the COBAPRESS process with, for example, the insertion of metal inserts into the preform, which is then forged. This has been defined in patent EP 586 314. The inserts are arranged once and for all in the preform and the final part is obtained.
Unlike foundry cores, inserts cannot be subsequently removed.
There thus is a major obstacle, for the discussed reasons.
In the context of the cast-forged technology, corresponding to the COBAPRESS process, the use of cores has been provided. For example, patent application PCT WO 2009/050382 provides the use of a salt or sand core, formed by means of a cold or hot box or “croning” inserted in a foundry preform to be then submitted to a preform forging operation. In practice, this patent application, which attempts to broaden the core forming mode, essentially refers to a core formed of sand or of resin, which corresponds to the initially-mentioned prior art. This core is actually provided with at least one gas discharge duct to discharge the gases out of the mold during the molding operation. Such gases, according to the Applicant of this patent application, may originate from the combustion of the resins or bonding agents contained in the core. Further, in this document, the gas discharge duct(s) help positioning the core in the mold in the molding operation. This thus generates a very specific structure, with technical constraints associated with this specific implementation and, in particular, specific means for the tightness of the preform blank.
Patent EP 850 825 also discloses the use of a core of lost material to form the hollow portion of a bicycle pedal crank. This core is continued by a support portion used to position the support inside of the foundry mold having the metal cast therein. The ensuing forging operation requires a previous partial removal of the core. This thus requires very specific operations with risks of leaving core fragments in the mold, which may be disturbing and create weak areas during the forging operation.
Patent application WO 84/04264 further discloses the use of salt cores used in foundry molding with a compaction effect (squeeze casting). In this case, the liquid metal is pressurized to 70 MPa as explained by the patentee at page 6, line 30, with a material in the liquid state. This pressure remains very low and does not correspond to a forging pressure, which approximately ranges from 600 to 700 MPa. This document is thus limited to the sole foundry application.
Based on the above considerations, the Applicant has searched for a solution capable of overcoming all the mentioned disadvantages and constraints of prior art.
The Applicant has followed a different approach from previously-described techniques, based on a new concept of foundry core manufacturing design capable of being implemented with no modification of the preform structure for the preform forging operation, and thus of the core surrounded with solid metal at pressures approximately ranging from 600 to 700 MPa.
The solution found and abundantly tested has enabled to validate the Applicant's choice for the manufacturing of this core.